1. Field of the Invention
The present invention relates generally to a logic timing diagram display apparatus, more specifically to an apparatus for displaying a logic waveform on a raster scan type display device.
2. Description of the Prior Art
A raster scan type display device is frequently used as a display apparatus, since that type of display device has the capability of displaying both timing diagrams and alphanumerics, and is flicker free even when a great deal of information is displayed. Moreover, a raster scan type display device is capable of reversing a monochromic display (black and white) with respect to the entire display or portions of the display, and also blinking all or selected portions of the display.
In a prior logic timing diagram display apparatus having a raster scan type display device such as a CRT, a logic waveform to be displayed on the CRT was considered to be a plurality of segments. All possible kinds of the logic waveform patterns which could occur in one display segment were stored in a waveform (Character) read only memory (ROM). The waveform patterns in the ROM were addressed in accordance with coded signals, referred to as FONT information. When the input logic signal was displayed as the timing diagram on the CRT, the input logic signal was first divided into a plurality of segments and each segment was converted into the appropriate coded signals, i.e., display codes or FONT information. This FONT information was stored in a display Random Access Memory (RAM) in sequential locations corresponding to their occurrence in the input logic signal waveform. The contents (FONT information) of the display RAM were read repeatedly in synchronism with the raster scan operation, and were applied to the address terminals of the waveform ROM for generating the waveform patterns by an intensity control signal. The CRT displayed the logic waveform in response to the intensity control signal and the raster scan operation.
These prior art systems have many disadvantages. One of them is that a large capacity ROM is needed, since all possible kinds of the waveform patterns for the display segment must be stored. A large capacity ROM is expensive. As the number of possible waveform patterns desired to be displayable increased, a much larger capacity was needed for the ROM. Another disadvantage was that the logic signal had to be processed since it had to be converted i.e. broken down into a series of display codes. Thus, it was difficult to display logic waveforms and use high speed horizontal scrolling, since the contents of the display RAM had to be changed in accordance with the scrolling. Moreover, the display RAM had to be rewritten if one wanted to magnify a portion of the currently displayed logic timing waveform.